1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of Related Art
An image forming apparatus as a full color copying machine has been used. Since the image forming apparatus becomes cheaper than a printer, the image forming apparatus is used instead of the printer to output printed objects, and the printed objects are cut out and bound by the image forming apparatus.
With the high definition of the image forming apparatus, an image of a document can be formed more accurately. Moreover, as sheet carrying precision when forming an image on a sheet or correction precision of an image forming position is improved, precision of the image forming position with respect to a sheet is also improved. For this reason, an additional image can also be formed on a sheet with high precision.
It has been proposed that an image of a chase pattern as an additional image is formed together with image data on a sheet. The chase pattern is a pattern that has relatively poor visibility and whose color cannot easily be identified by human eyes. The chase patterns are unique patterns assigned to each image forming apparatus. Printing the chase patterns on a sheet prevents the printed object from being altered. In this regard, it has been proposed that positions of the chase patterns are varied on the sheets to enhance the prevention effect of the alteration (for example, refer to Japanese Patent Application Laid-open No. 7-87297).
Unfortunately, when forming images on a plurality of sheets by the image forming apparatus, the chase patterns may appear at edges of the sheets if a full bleed image is formed or sheet cutting is performed. The chase pattern dots may be noticeable as continuous stripes when taking a side view of a stack of the sheets.
Referring to FIGS. 19A and 19B, a specific example of an image forming processing in which stripes appear, will be explained. FIG. 19A shows sheets P81 to P84 on each of which chase pattern dots D8 constituting a chase pattern are formed. FIG. 19B shows the sheets P81 to P84 on each of which the chase pattern dots D8 are formed, being stacked. Hereinafter, the chase patterns may be referred to as chasing marks and the chase pattern dots may be referred to as chasing mark dots.
Suppose that the chasing mark dots D8 are formed at edges of the sheets P81 to P84 as shown in FIG. 19A. The chasing mark dots D8 are located at an edge of the sheet P81, for example. In this case, as shown in FIG. 19B, stripes E1 to E3 formed by the chasing mark dots D8 appear when the sheets P81 to P84 are stacked.
FIG. 20 shows a sheet P9 on which chasing marks CM9 are simply shifted. As shown in FIG. 20, suppose that the chasing marks CM9 on the sheet P9 are simply shifted in an image valid area F9, and the sheet P9 is cut out at sheet cutting positions L8 and L9. The shifted chasing mark CM9 may be located on the sheet cutting position L9. In this case, if the sheets P9 after the sheet cutting are stacked, stripes formed by the chasing marks may appear at a side of a stack of sheets P9.